Acceptable handling of audio signals with very large dynamic range presents significant challenges to audio amplification and processing circuits and systems, in particular for audio amplification and processing circuits targeted for portable devices and applications such as mobile terminals, hearing instruments, headsets, sound recording cameras etc.
Since portable devices are powered from battery sources severe constraints as to a maximum acceptable power consumption of the audio amplification circuit is typically imposed. To further worsen the situation, there typically exist similar constraints on a maximum DC supply voltage that can be provided to the audio amplification and processing circuitry. The audio amplification and processing or conditioning circuitry often comprise preamplifiers, analogue-to-digital converters, active filters, voltage supply regulators, etc. The maximum DC power supply voltage, and therefore AC signal voltage swing, will often be limited to a voltage below a maximum rated voltage of the particular semiconductor process used to implement the signal processing or conditioning circuitry on. Furthermore, a continuing trend of shrinking minimum feature sizes of active devices on semiconductor dies and circuits in general and CMOS processes in particular, leads to a constant decline of the maximum DC power supply voltage these active devices can withstand or tolerate. Audio amplification systems and circuitry, such as audio signal controllers and audio amplification circuits, which can operate on these declining DC power supply voltages without audio performance degradation, are therefore highly desirable. It is generally undesirable to reduce performance of the audio amplification system, for example by lowering dynamic range or amplification of a preamplifier, to accommodate large audio input signals despite the decrease of the DC power supply voltage. The DC power supply voltage may be less than 2 Volt or even less than 1.5 Volt. The audio amplification system should therefore be able to provide unimpaired audio quality at low level signals and at high level signals at the decreased or lowered DC power supply voltage.
An interesting application of the present audio amplification system is to amplify and digitize audio signals in miniature microphones where microphone transducer elements are capable of generating audio input signals with a very large dynamic range. The microphone transducer elements may comprise a capacitive electret or condenser transducer of a miniature ECM, that is capable of handling very high sound pressure levels and generate correspondingly large transducer signals without significant distortion. These very high sound pressure levels, for example peak sound pressure levels above 110, 120 or 130 dB SPL, can originate from different types of acoustic sources for example car door slamming, wind noise and augmented live music performances. However, prior art microphone amplification systems have not been capable of handling the entire dynamic range of these transducer signals in an entirely satisfactory manner, e.g., without increasing equivalent input noise of the miniature microphone or overloading the miniature microphone at large sound pressure levels or both.
Accordingly, there is a need in the art for microphone amplification circuits and systems capable of handling the entire dynamic range of the transducer signals generated by microphone transducer elements, or other audio source signals with large dynamic range, without excessive distortion or noise within the previously discussed constraints on DC power supply voltage and power consumption dictated by portable or battery-powered devices and applications. The present audio amplification circuit exploits a dual-preamplifier structure wherein a first preamplifier can handle signal amplification at low and normal audio input signal levels and a second preamplifier can handle signal amplification at very high audio input signal levels. A distortion compensation network is adapted to supply a distortion compensation signal from a first input of the first preamplifier to a second differential input of the second preamplifier such that distortion in the output signal of the second preamplifier is cancelled or attenuated during conditions with large levels of the audio input signal. The large level of the audio input signal may correspond to a peak sound pressure level above 110, 120 or 130 dB SPL on a microphone coupled to the audio amplification circuit.